Survival in a bad neighborhood: pancreatic islets in cystic fibrosis

Abstract

In cystic fibrosis (CF), ductal plugging and acinar loss result in rapid decline of exocrine pancreatic function. This destructive process results in remodeled islets, with only a modest reduction in insulin-producing β cells. However, β-cell function is profoundly impaired, with decreased insulin release and abnormal glucose tolerance being present even in infants with CF. Ultimately, roughly half the CF subjects develop diabetes (termed CF-related diabetes (CFRD)). Importantly, CFRD increases CF morbidity and mortality via worsening catabolism and pulmonary disease. Current accepted treatment options for CFRD are aimed at insulin replacement, thereby improving glycemia as well as preventing nutritional losses and lung decline. CFRD is a unique form of diabetes with a distinct pathophysiology that is as yet incompletely understood. Recent studies highlight emerging areas of interest. First, islet inflammation and lymphocyte infiltration are common even in young children with CF and may contribute to β-cell failure. Second, controversy exists in the literature regarding the presence/importance of β-cell intrinsic functions of CFTR and its direct role in modulating insulin release. Third, loss of the CF transmembrane conductance regulator (CFTR) from pancreatic ductal epithelium, the predominant site of its synthesis, results in paracrine effects that impair insulin release. Finally, the degree of β-cell loss in CFRD does not appear sufficient to explain the deficit in insulin release. Thus, it may be possible to enhance the function of the remaining β-cells using strategies such as targeting islet inflammation or ductal CFTR deficiency to effectively treat or even prevent CFRD.

Keywords: cystic fibrosis; diabetes; inflammation; insulin secretion; β-cell mass.

Figure 1.

Pancreas morphology (haematoxylin staining) in a non-CF control (A; 17 years old), CF-no diabetes (B; 18 years) and CFRD (C; 46 years) autopsy human pancreas specimen. Extensive fatty replacement and fibrosis are evident in the CF and CFRD cases. Islets, visualized by insulin immunohistochemistry (brown), appear remodeled but clearly present in CF and CFRD even when compared to non-CF control. Scale bar = 100 μm (Scale bar in A applies to all panels).

Figure 2.

CFTR expression in isolated human islet cells. Data captured from [(Segerstolpe, et al. 2016) as visualized at http://sandberg.cmb.ki.se/pancreas/]. (A) Single cell RNAseq results for CFTR expression in isolated islets collected from 10 individuals. Boxplots summarize CFTR expression in Reads Per Kilobase of transcript per Million reads (RPKM) on a log2 scale across the 7 major cell types for each donor. The first 6 boxes for each cell-type correspond to healthy individuals and the last 4 to individuals with type 2 diabetes. (B) t-SNE plot of cells grouped by transcriptome similarity and colored according to CFTR expression according to the indicated log2 RPKM scale. (C) Legend indicating cell types. (B, C) Left panels show all cell types (n=2,209) and right panels show endocrine cell types only (n=1,554).

Figure 3.

Presence of cytokeratin 7-positive duct cells (red) in isolated human islets, immediately upon receipt from the IIDP. Cytokeratin 7 immunoreactivity was detected using the anti-Cytokeratin 7 antibody EPR1619Y from AbCam (catalogue no. ab68459) based on the approach described in (Sun et al. 2017).

Figure 4.

CFTR-knockdown in wild-type adult ferret and human islets reduces glucose stimulated insulin secretion. (A) Infection of adult ferret islets with a fCFTR-shRNA expressing recombinant adenovirus (Ad.fCFTR-shRNA) that contains a CMV-driven EGFP transgene in the E3 region of the virus. Fluorescent photomicrographs of mock and Ad.fCFTR-shRNA infected islets are shown. (B) Evaluation of fCFTR and housekeeping (PPIB) RNA levels in CF and WT adult ferret islets 48 hrs following Ad.fCFTR-shRNA or Ad.Scr-shRNA (scrambled control) infection. Plots represent mean±SEM for N experiments; ** P<0.01, two-tailed student’s t-test. (C) Photomicrograph of adult ferret islets 48 hours following infection with Ad.fCFTR-shRNA and Ad.Scr-shRNA. (D, E) Insulin secretion in response to 1.67 mmol/l glucose (G1.67) or 16.7 mmol/l glucose (G16.7) by adult WT ferret islets (D) and human islets (E) 48 hrs following infection with Ad.fCFTR-shRNA, Ad.hCFTR-shRNA, or Ad.Scr-shRNA as indicated. (D,E) Graphs show the mean±SEM for the N independent batches, with dots showing the mean per batch performed on 1–4 replicates. Statistical analysis comparing Scr-shRNA versus CFTR-shRNA within each glucose category was performed by mixed effects modeling treating batch replicates as repeated measures using the R package nlme; *** P≤0.0001.